Electrical resistor and method of making same



July 26, 1955 G. N. HowATT 2,714,148

Filed Jan. 28, 1952 Pedara/br Coe/ef1 Caffe/1 Carrel4 INVENTOR.

Lm f' @Maf -f States Patent Office 2,714,148 Patented July 26, 1955 ELECTFJCAL RESISTOR AND METHOD OF MAKING SAME Glenn N. iliowatt, Metuchen, N. J., assigner to Gulton gifg. Corp., Metuchen, N. J., a corporation of New ersey Application January 28, 1952, Serial No. 268,490 l1 Claims. (Cl. 201-63) This invention is directed to an electrical resistor and the method of making the same. While the resistor of this invention is capable of substantially any use it is particularly adaptable for use with printed electrical circuits.

In printed electrical circuits, being now more and more widely used, because of cost and speed considerations, the conductors of the electrical circuits are printed on a plate of insulating material and the electrical resistors forming parts of the electrical circuits are also commonly printed thereon as by squeegeeing or spraying electrical resistance material thereon. lf too much or too little electrical resistance material is applied to the circuit so as to provide incorrect resistance values, this is not usually detected until the printed circuit is substantially completed and tested. When this occurs, either the plate with its circuit components is discarded or the electrical resistance material must be manually scraped away or added to correct the electrical resistance values thereof. This procedure is, therefore, time consuming and expensive and presents one of the major obstacles in the manufacture of such printed electrical circuits.

The principal object of this invention is to provide an improved electrical resistor which is particularly adaptable for use with such printed electrical circuits, which is simple and inexpensive to manufacture, which may be tested and calibrated before application to the printed electrical circuit and which may be quickly, simply and inexpensively applied to such printed electrical circuits.

Another object of this invention is to provide a method for making said improved electrical resistor and for applying the same to such printed electrical circuits,

A further object of this invention is to provide improved electrical resistors which are adaptable for general use and also to provide a method for making the same.

Briey, in accordance with this invention finely divided electrically conductive particles are mixed with and uniformly dispersed in a heat liqueiable thermosetting-resin and an accelerator is also applied to this mixture. The proportions of the mixture are dependent upon the ultimate electrical resistance characteristics desired. The mixture is then deaerated and cooled to form a plastic mass. This plastic mass is then preferably extruded into an elongated plastic, but form retaining, rod having a cross-sectional area also dependent upon the ultimate electrical resistance characteristics desired. This plastic rod is then cut into sections to form plastic, but form retaining, partially cured electrical resistor elements. These resistor elements may be then tested and calibrated and selected for their electrical resistance characteristics, account being taken for subsequent change in electrical resistance characteristics caused by possible shrinkage of the electrical resistor elements as they are cured into the iinal electric resistor.

These plastic partially cured electrical resistor elements, already tested arid calibrated, then may be applied to the plate of insulating material having the electrical printed thereon, for example, with the resistor element bridging a pair of spaced conductors across which an electrical resistance path is to be established. This plastic partially cured electrical resistor element is cured in place on the plate, letting the softening or melting of the recircuit sistor element bond the same to the plate and the electrical conductors. Alternatively, the ends of the partially cured electrical resistor element may be melted by applying heat thereto to stick them to the pair of electrical conductors and then the resistor element cured. When the electrical resistor element is adequately cured, it becomes a solid electrical resistor which is firmly bonded to the electrical conductors and the plate of insulating material supporting the same. In this way electrical resistors may be quickly and inexpensively manufactured, tested and calibrated and quickly, inexpensively and permanently applied to printed circuits in a manner to avoid the aforementioned objections to present day practice.

If desired, the partially cured plastic resistor element may take the form of a pencil for writing an electrical resistance path on the insulating plate between the pair of electrical conductors and when the electrical resistance path so formed is completely cured it will become solid and firmly bonded to the electrical conductors and the plate. Such a pencil may also be used for correcting present day printed resistors of low value.

Instead of applying the plastic partially cured resistor elements to the electrical conductors of a printed circuit, the resistor elements may have electrical connections secured to their ends to form electrical resistors for general use. Here electrical connections in the form of metal connectors, cups or the like may be applied to the ends of the resistor elements and when the latter are completely cured the electrical connections will be firmly bonded thereto.

Other objects and advantages of this invention will become apparent to those skilled in the art upon reference to the accompanying specification, claims and drawings, in which:

Fig. l is a flow diagram illustrating the method for making the improved electrical resistor of this invention;

Fig. 2 is an elevational view of a plastic semicured extruded rod from which the electrical resistor elements are made;

Fig. 3 is an end elevational view of Fig. 2;

Fig. 4 is an elevational view of a plurality of partially' cured plastic electrical resistor elements obtained by cut-- ting the rod of Fig. 2;

Fig. 5 is a top plan view showing an electrical resistor element of Fig. 4 applied to a pair of spaced electrical conductors printed on a plate of insulating material;

Fig. 6 is a side elevational View of Fig. 5;

Fig. 7 is an end elevational view of Fig. 5;

Fig. 8 is a side elevational view of an electrical resistor having electrical connections thereon for general use;

Fig. 9 is an end elevational view of Fig. 8;

Fig. l0 is a view similar to Fig. 8 but showing the electrical connections to be cup shaped in configuration; and

Fig. ll is a side elevational view of a pencil formed from the plastic semicured rod of Fig. 2 for writing resistance paths on a plate.

Referring iirst to the flow diagram of Fig. l, finely divided electrically conductive particles are mixed in a suitable mixer with a heat liqueiiable thermosetting resin in substantially liquid form. The finely divided electrical'- ly conductive particles are uniformly dispersed in the heat liquefable thermosetting resin by the mixer. The electrically conductive particles may be of substantially any finely divided metal or carbon having electronically conductive characteristics; preferably powdered graphite is utilized since it may be readily uniformly dispersed and maintained in dispersed condition. rl`he heat liqueiable thermosetting resin may be of any standard type, such as phenol-formaldehyde, urea-formaldehyde or the like. The proportions of the electrically conductive par- 3 ticles and the heat liqueiiable thermosetting resin are dependent upon the desired electrical resistance characteristics of the ultimate electrical resistor.

Either during mixing or after mixing an accelerator is added to the mixture. The accelerator may be of any conventional type, such as ammonium hydroxide, sodium carbonate, hydrochloric acid or the like. The purpose of the accelerator is to hasten curing of the thermosetting resin of the mixture. The proportions of the accelerator utilized in conjunction with the thermosetting resin may follow standard practice.

The resultant mixture in substantially liquid form is deaerated by a deaerator, or other suitable mechanism to eliminate air from the mixture, to provide a dense mixture. This deaerated mixture is then cooled in a suitable cooler or other apparatus to a plastic state Wherein, at least at the cooled temperatures, it may be extruded and substantially hold its shape. This plastic mass is then extruded into a plastic, but form retaining, rod 10, as illustrated in Figs. 2 and 3. The cross-sectional area of the plastic rod is dependent upon the electrical resistance characteristics desired in the ultimate electrical resistor produced by this method.

The extruded rod 10 may then be cut into sections 11, as illustrated in Fig. 4, to form electrical resistor elements 11 comprising a plastic, but form retaining, partially cured heat liquefiable thermosetting resin having the accelerator therein and the nely divided electrically conductive particles uniformly dispersed therein. The lengths of these plastic partially cured resistor elements are also dependent upon the ultimate electrical resistance characteristics desired. These electrical resistor elements are then tested in a suitable tester for measuring and determining the electrical resistance characteristics thereof. In so doing, account is taken for subsequent change in electrical resistance characteristics caused by possible shrinkage of the electrical resistor elements as they are cured into the final electrical resistor. As a result of this testing the electrical resistor elements may be selected in accordance with their electrical resistance characteristics and, if desired, they may be shortened by cutting if their electrical resistance values are not correct. This testing of the electrical resistance characteristics of these electrical resistor elements 11 at this stage in the process is important in that the elements may be selected, calibrated or discarded with a minimum of expense.

These electrical resistor elements 11, already tested and calibrated, may then be assembled as, for example, to a printed electrical circuit, as illustrated in more detail in Figs. 5 to 7. Here, by way of illustration, a pair of spaced electrical conductors 12, forming a portion of a printed electrical circuit, are shown to be printed in the usual fashion on a plate 13 of electrical insulating rnaterial. The plastic partially cured electrical resistor elements 11 are then applied to the plate 13 bridging the electrical conductors 12 for the purpose of completing an electrical resistance path thereacross. This plastic partially cured electrical resistor element 11 is then cured in place on the plate 1-3, letting the softening or melting of the resistor element bond the same to the plate and the electrical conductors. When the electrical resistor element 11 is completely or adequately cured, and this curing is hastened by the accelerator, it becomes a solid electrical resistor which is rmly bonded to the electrical conductors and the plate. In this way the nal solid electrical resistor is firmly and permanently secured to the plate 13 of insulating material and the electrical conductors 12 to form an integral part of. the printed circuit and since the electrical resistor has already been tested and calibrated, there is no need to alter the resistance value thereof or to discard the printed electrical circuit because of incorrect resistance values.

Instead of applying the plastic partially cured electrical resistor elern'ent 1 1 to the electrical conductors 12 in the manner described above, the ends of the 'electrical resistor element may be melted or softened by applying heat thereto in order to stick the electrical resistor element to the pair of electrical conductors. When the electrical resistor element so secured to the electrical conductors is cured, it becomes rmly and permanently bonded to the electrical conductors. The melting or softening of the plastic partially cured electrical resistor element, of course, is controlled so as to prevent substantial change in the length and cross-sectional area thereof so as not to alter the electrical resistance characteristics thereof.

As illustrated in Figs. 8, 9 and 10, electrical connectors may be secured to the electrical resistor element to form an electrical resistor for conventional use. In Fig. 8 the plastic semicured electrical resistor element 11 may have its ends heated to melt the same so that electrical connectors 14 may be stuck thereto. When the electrical resistor element 11 is completely cured, the electrical connectors 14 are permanently bonded thereto to form electrical connections for the electrical resistor. The electrical connectors 14 may have tabs 15 to which may be soldered conventional leads. In Fig. l0 cup shaped electrical connectors 16 are shown to be applied to the ends of the electrical resistor element 11. Here the ends of a plastic semicured electrical resistor element may be received in the cup shaped electrical connectors 16 and heated to stick the same together. Here again when the electrical resistor element 11 is completely cured the electrical connectors 16 will be rmly and permanently bonded thereto.

The plastic rod 10 of Fig. 2 may be formed into a plastic pencil 17, as illustrated in Fig. 1l. This partially cured plastic pencil may then be utilized for Writing an electrical resistance path on the insulating plate between the pair of electrical conductors. When the electrical resistance path so written onto the plate is completely cured, it will become solid and will be firmly bonded to the electrical conductors and the plate. Such a pencil may also be used for correcting present day printed resistors of low value.

As further examples of this invention, the heat liquefiable thermosetting resin from which the electrical resistors are made may be of the type of ethary resins such as sold by Ciba Company, Inc. under the mark Araldite. The finely divided electrically conductive particles dispersed therein may comprise graphite, such as Dixon graphite 200a09 sold by Joseph Dixon Crucible Co., or Statex carbon black made from burning oil such as sold by Columbian Carbon Co., or Continental AA carbon black made from burning natural gas such as sold by Continental Carbon Co. The kinds and quantities of the nely divided electrically conductive particles disposed in the thermosetting resin depends upon the resistance values desired. The cross-sectional areas of the resistors made from these materials may correspond to those of circular rods having .010 to .025 inch diameters and the lengths thereof may vary from 1A; inch to 1 inch, the normal being about Mz inch. Generally the wattage rating of the resistor governs the diameter thereof which in turn also greatly affects the composition of the resistor. The following chart is generally based on resistors of .020 inch in diameter and of 1/2 inch long and rated at 1/z watt:

Resistance Value Rtssm Material 200 ohms.. 2. 5 1 pt. Dixon graphite 20D-09. 500 ohms.. 4 Do. 1,000 ohms.. 5 Do. 2,000 ohms.. 6 Do. 5,000 ohms.. 4 1 pt. Statex carbon black. 10,000 ohms... 5 Do. 20,000 ohms. 6 Do. 50,000 ohms 7. 5 Do. 100,000 ohms 8.5 Do 500,000 ohms 7. 5 Do. 1 Meg 8 1 pt. Continental AA carbon black, 2 Meg. 9 Do. 5 Meg 12 Do.

These figures are somewhat approximate but illustrative. For greater diameter resistors, higher resistance mixes are required for any given value of electrical resistance.

While for purposes of illustration several embodiments of this invention have been disclosed, other embodiments thereof may become apparent to those skilled in the art upon reference to this disclosure and, therefore, this invention is to be limited only by the scope of the appended claims.

l claim as my invention:

l. In a printed electrical circuit having a plate of electrical insulating material and a pair of spaced electrical conductors printed thereon, a solid synthetic thermosetting resinous body having finely divided electrically conductive particles uniformly dispersed therein and bonded to the plate and the pair of spaced electrical conductors to provide an electrical resistance path between the pair of spaced electrical conductors.

2. An electrical resistor consisting essentially of a plastic, but form retaining, partially cured heat liquefiable thermosetting resinous body having an accelerator therein and finely divided electrically conductive particles uniformly dispersed therein and adapted upon heating to be bonded to a pair of electrical connections and solidified.

3. An electrical resistor, for a printed electrical circuit having a plate of electrical insulating material and a pair of spaced electrical conductors printed thereon, consisting essentially of a plastic, but form retaining, partially cured heat liquetiable thermosetting resinous body having an accelerator therein and finely divided electrically conductive particles uniformly dispersed therein and adapted upon heating to be bonded to the plate and the pair of spaced electrical conductors and solidified to provide an electrical resistance path between the pair of spaced electrical conductors.

4. A resistor element for forming electrical resistors consisting essentially of a plastic, but form retaining, partially cured heat liquetiable therrnosetting resinous body having an accelerator therein and nely divided electrically conductive particles uniformly dispersed therein.

5. A pencil resistor element for forming electrical resistors comprising a pencil shaped plastic, but form retaining, partially cured heat liquefiable thermosetting resinous body having an accelerator therein and finely divided elec-- trically conductive particles uniformly dispersed therein and adapted to deposit a portion thereof on an object When stroked thereover to form an electrical resistance path.

6. rThe method of forming electrical resistors comprising, mixing and uniformly dispersing finely divided electrically conductive particles in a heat liquetiable thermosetting resin, mixing an accelerator into the mixture, cooling the resultant mixture to a plastic state, forming the plastic mixture into plastic, but form retaining, partially cured electrical resistor elements, applying electrical conductors to the plastic partially cured resistor elements, and completely curing the electrical resistor elements into solid electrical resistors with the electrical conductors bonded thereto.

7. The method of forming electrical resistors comprising, mixing and uniformly dispersing finely divided electrically conductive particles in a heat liquefiable thermosetting resin, mixing an accelerator into the mixture, cooling the resultant mixture to a plastic state, forming the plastic mixture into plastic, but form retaining, partially cured electrical resistor elements, testing the plastic partially cured electrical resistor elements for electrical resistance characteristics, applying electrical conductors to the plastic partially cured resistor elements, and completely curing the electrical resistor elements into solid electrical resistors with the electrical conductors bonded thereto.

8. The method of applying an electrical resistor to a printed electrical circuit having a plate of electrical insulating material and a pair of spaced electrical conductors printed thereon comprising, forming an electrical resistor element of a plastic, but form retaining, partially cured heat liquefiable thermosetting resin having an accelerator therein and finely divided electrically conductive particles uniformly dispersed therein, applying the electrical resistor element to the plate across the pair of spaced electrical conductors, and completely curing the electrical resistor element into a solid electrical resistor bonded to the plate and to the pair of spaced electrical conductors.

9. The method of applying an electrical resistor to a printed electrical circuit having a plate of electrical insulating material and a pair of spaced electrical conductors printed thereon comprising, forming an electrical resistor element of a plastic, but form retaining, partially cured heat liquefiable thermosetting resin having an accelerator therein and finely divided electrically conductive particles uniformly dispersed therein, testing the partially cured plastic electrical resistor element for its electrical resistance characteristics, applying the tested electrical resistance element to the plate across the pair tor bonded to the plate and to the pair of spaced electrical conductors.

l0. The method of forming an electrical resistor comprising, forming an electrical resistor element of a plastic, but form retaining, partially cured heat liquefiable thermosetting resin having an accelerator therein and finely divided electrically conductive particles uniformly dispersed therein, applying electrical conductors to the plastic partially cured resistor clement, and completely curing the electrical resistor element into a solid electrical resistor with the electrical conductors bonded thereto.

ll. The method of forming an electrical resistor comprising, forming an electrical resistor element of a plastic, but form retaining, partially cured heat liquefiable thermosetting resin having an accelerator therein and finely divided electrically conductive particles uniformly dispersed therein, testing the partially cured plastic electrical resistor element for its electrical resistance characteristics, applying electrical conductors to the tested plastic partially cured resistor element, and completely curing the electrical resistor element into a solid electrical resistor with the electrical conductors bonded thereto.

References Cited in the file of this patent UNiTED STATES PATENTS OTHER REFERENCES Brunetti et al.: Nat. Bureau of Std. Circular #468 on Printed Circuit Techniques issued November l5, 1947. 

